This invention relates to a fluorescent material prepared from lanthanum cerium orthophosphate activated by terbium (Tb) and a fluorescent lamp provided with said fluorescent material.
The conventional Deluxe type lamp whose color temperature ranges from the warm white to the daylight comprises a mixture of several kinds of fluorescent materials for the object of providing a flat spectral distribution similar to that of the sunlight. The prior art Delux type lamp based on this concept had a high color rendition, but had the drawback that a light flux unavoidably dropped. A large number of commercially available fluorescent lamps whose bright membrane was prepared from a fluorescent material of calcium halophosphate was intended to assure a high light flux at the sacrifice of color rendition. In recent years, however, demand is made for improvement of the quality of an illumination light. The problem at present is how to elevate color rendition while a high light flux is retained.
To resolve this problem, there has been proposed a fluorescent lamp comprising a 3-wavelength type assembly of fluorescent materials. This proposed fluorescent lamp is formed of an assembly of fluorescent materials respectively emitting a blue light having a wavelength of approximately 450 nm, a green light having a wavelength of approximately 540 nm and a red light having a wavelength of approximately 610 nm. Therefore, said proposed fluorescent lamp can elevate color rendition to as high a level as about Ra85, while maintaining about the same light flux as that of the aforesaid calcium halophosphate fluorescent material used with the commercially available fluorescent lamp.
The above-mentioned 3-wavelength type fluorescent lamp is already marketed. The most important one of the three fluorescent materials emitting blue, green and red lights is the type issuing a green light. The more efficient this green light-emitting material, the larger the volume of light fluxes obtained. At present, the development of this green light-emitting material is advanced.
The green light-emitting material is used alone with a green light-emitting fluorescent lamp of an electronic copying machine. In this case, such a fluorescent material is demanded and is little subject to deterioration with time in the luminosity, even when applied under the rigorous conditions in which intermittent illumination is carried out at high frequency.
Green light-emitting fluorescent materials known to date include (a) Tb-Ce activated magnesium aluminate, (b) Tb-Ce activated yttrium silicate, and (c) Tb-activated yttrium silicate. All these green light-emitting fluorescent materials have a highly efficient quality, but still are accompanied with some defects. The (a) fluorescent material whose matrix is aluminate has to be baked at an extremely high temperature as 1350.degree. to 1500.degree. C. to render it of high quality. Further, said matrix has to be grown by baking of long hours. Consequently, the quantity production of said (a) fluorescent material involves the provision of large scale equipment and the consumption of a large amount of cost.
With the (b) and (c) fluorescent materials, the content of terbium raises a problem. Since the matrix of the (b) and (c) is a silicate, the content of terbium should be as large as at least 30 mol% per molecular weight of said silicate. In other words, about 180 g of Tb.sub.4 O.sub.7 should be used per kg of said silicate. Since this Tb.sub.4 O.sub.7 is an expensive one among the commonly used rare earth materials, the above-mentioned fact unavoidably increases the cost of a fluorescent material and consequently harmfully affects the cost of the resultant fluorescent lamp.